A two-component developer for use in an electrophotographic process is composed generally of a toner and a carrier. The carrier is used for the purpose of giving a proper polarity and a proper amount of triboelectric charge to the toner.
As the carrier, in order to improve its durability and triboelectric chargeability, there is used a resin-coated carrier prepared by coating a resin on the surface of core particles.
Known as the resin-coated carrier are those comprising magnetic particles each having on the surface thereof a fluoro-resin coat phase in order to prevent the adhesion of a toner material to the carrier surface and to render positive triboelectric charge to the toner as disclosed in JP O.P.I. Nos. 208754/1983, 176048/1985, 16617/1985 and 240758/1984.
As means for forming the resin coat layer, a wet spray coating method and a dry coating method are widely known. There is also known a method in combination of both as disclosed in JP O.P.I. No. 13972/1990.
The above technique is for carrier production improvement in which a resin intermediate phase is provided on a carrier core by a dry coating process, and on the intermediate phase is provided a triboelectric charging control resin layer.
However, the carrier needs to be still further improved. The resin used in the triboelectric charging control phase is not sufficient in the adhesion property to the intermediate phase, and the phase is liable to be worn out by the force from the stirring inside the developer vessel, developer carrier, developer regulation member, and the like, so that it is insufficient in the durability. In addition, the triboelectric charge largely changes according to the changes in temperature between 10.degree. C. and 30.degree. C., so that it is difficult to obtain constantly stable images over a long period under various environmental conditions.